The present invention generally relates to storage devices and data backup methods associated with such storage devices, and, more specifically, to data backup technology in context of a storage apparatus accessed from a host apparatus connected via a network.
In order to prevent the corruption or loss of data in a computer system, it is important to periodically back up the data stored in the storage apparatus. The backup of data is typically achieved by periodically copying the whole or part of a data volume (hereinafter simply referred to as “volume”). Such copies are maintained at a different storage site, thereby resulting in creating a replication volume (backup data) of the primary volume. When data on the volume is damaged or lost, the state of the data volume before such loss can be restored by restoring the backed up data to its original location.
Snapshot is also known as technology used in the backup of data. The snapshot is a technique for storing a data image of a volume at a certain point in time. For example, even when data is lost due to a failure, the state of the volume before such loss can be restored by referring to the stored snapshot image. Using the snapshot technique, one can maintain the state of a volume at a certain point in time more efficiently than using the method of creating a replication volume by simply copying the entity of data.
As one example of using the snapshot technology, Japanese Patent Laid-Open Publication No. 2001-306407 discloses providing one of the independent redundant storage extents to a computer for normal access, and providing the other storage extent for acquiring a snapshot image.
Because the data consistency will be lost if the data is rewritten during the backup procedure of the data to be maintained, it is necessary to guarantee that the rewriting of data will not be performed during the backup procedure. Therefore, in view of data consistency, after inspecting the usage status of volumes in a period of time when volumes are not generally accessed; for instance, late at night and on holidays, data is copied to a volume of an unused status.
For purposes of backup scheduling, the time frame in which data can be backed up is referred to as a backup window. For the backup of data, all processing routines from start to finish need to be performed during the backup window.
Because data backup processing requires a certain amount of time, there may be cases where backup processing cannot be completed within the backup window, especially when the backup window is small or the number of volumes (or the amount of data) to be backed up is large. Thus, Japanese Patent Laid-Open Publication No. 2002-297427 discloses performing backup scheduling based on the importance of data to be maintained or the reliability of the storage apparatus.
With backup processing, because only the volumes that were backed up until a certain point in time can be restored, if the period from completing the backup to starting the subsequent backup is long, data that was rewritten during such period cannot be restored. In contrast, if backup is frequently performed, not only will the system performance deteriorate, this will cause increased costs associated with the backup. RPO (Recovery Point Objective) is an index showing the frequency of backup processing, and the method of setting the RPO relates to the operational costs of the storage apparatus.
Upon installing and operating a storage apparatus, reduction of costs for backup is desirable. In a system using a storage apparatus that provides one or more volumes to a plurality of personal computers, for purposes of reducing costs for backup, for example, RPO can be set to be performed once a day. In addition, a single backup module can be used for performing backup processing to as many volumes as possible.
Nevertheless, with a system that follows the aforesaid RPO backup policy, for example, when there is an act of invading the backup window such as in an exceptional usage where a user operates the computer until late at night and boots such computer early in the morning, the usage will not conform to a predetermined backup schedule, and a so-called “incomplete” backup may occur. Further, when one backup module performs the backup of numerous data volumes, there is a possibility that processing cannot be completed within the backup window depending on the data volume to be backed up.